This invention pertains to the art of continuous metal strip annealing and, more specifically, to the art of preheating furnaces which are adapted to clean and to preheat the strip prior to a subsequent further treating in an annealing furnace. In particular, this invention pertains to the class of preheating furnace used in continuous steel strip non-ferrous metal coating processes such as employed in galvanizing and aluminum coating lines.
The lengths of the pass lines in preheating and annealing furnaces are a function of the temperatures of the respective furnaces and of the speed of the strip passing through these furnaces. It is customary in the prior art for a preheat furnace to receive strip at ambient temperature and to preheat it to within a few hundred degrees of the desired annealing temperature. The strip then passes into the annealing furnace, which is of sufficient length to bring the strip up to full annealing temperature and to perform the desired annealing operation on the strip. Thus, the longer the annealing furnace pass line, the more time is available to bring the strip up to full annealing temperature. However, if space considerations require that the annealing furnace be shortened, then this may be compensated for by increasing the temperature of the strip to more closely approximate the annealing temperature before it leaves the preheat furnace. The disadvantage of higher operating temperatures in preheat furnaces is that the life of the ceramic insulation is foreshortened by the higher temperature at which the furnace is operated.
Experience in operating preheat and annealing furnaces has developed certain accepted optimum operating temperatures for preheat furnaces and for annealing furnaces, taking into consideration the various compromises which must be made to obtain reasonable efficient furnace operation. By operating the preheat furnace substantially within a range of from 2200.degree. F. to 2400.degree. F., with a pass line of sufficient length, the exit temperature of the strip may be regulated to substantially 1000.degree. F. At this temperature the strip passes into the annealing furnace where it is further elevated a few more hundred degrees to the desired annealing temperature and held at that temperature for the required length of time before exiting the annealing furnace for further processing.
Considering the preheat furnace and the annealing furnace as a unit, these furnaces are traditionally designed for constant tonnage production. Constant tonnage is used in the context that at optimum strip line speed and with optimum furnace operating temperatures, the furnaces are capable of heat treating a predetermined maximum number of tons of steel per hour. However, such operation is generally limited within a range of strip thickness gauges in that, if the strip is over a certain gauge the furnace will not be able to heat the strip to the required temperature within the required time. Conversely, if the strip is below a certain specified gauge then the furnace is incapable of moving the strip through the furnace fast enough to keep pace with the heating capacity of the furnace. Accordingly, it has been customery to have different sizes of furnaces designed to process steel strip within specified gauge ranges. Thus, furnaces designed to process very narrow gauge strips would usually be incapable of efficiently handling the higher gauge strips. It also follows, therefore, that furnaces designed to process higher gauge strips cannot efficiently process narrow gauge strips.